Intercommunication system



Aug. 28, 1956 HARDY INTERCOMMUNICATION SYSTEM 2 Sheets-Sheet 1 FiledJan. 9, 1951 INVENTOR Rene Ila/1 By I g j ATTORNEYS United States PatentINTERCOMIVIUNICATION SYSTEM Ren Hardy, Paris, France, assignor to Sepco,Paris, France Application January 9, '1951,=Serial No.-205,052

4 Claims. (Cl. 179-1) As conducive to an understanding of the inventionit is to be noted that where in order to afford communica-- tion betweentwo positions, illustratively a local and remote station, a microphoneand speaker are placed near each other at the local station andconnected respectively through suitable circuits to a speaker andmicrophone at a remote station, which each of the respective circuitshaving amplifying means associated therewith, the amplitude of the soundemitted by each of the speakers may be raised to any suitable amount.However, as the sounds emitted by the speaker at one station will bepicked up by the adjacent microphone and transmitted to' the speaker atthe other station from which the sounds originated and as the soundsfrom such speaker will also be picked up by the microphone adjacentthereto, the original sounds will be built up as the cycle is repeatedto the limit of the range of the amplifiers. The resultant oscillationscause a howl known as acoustic feed-back, which will drown out theconversation and render the conversation.

Where, in order to prevent such acoustic feed-back,

the microphone at each station is normally cut' OE and:

requires the pressing of a key for it to be used, the results areunsatisfactory in that it requires an aflirmative ac-,'

tion on the part of the user each time he speaks and if he shouldinadvertently fail to press the key when talking into the microphone, nosound would be transmitted.

Where systems are used which automatically out OK an amplifierassociated with one of the microphonespeaker combinations when themicrophone associated with the other amplifier is spoken into and suchauto-' matic cut-cit is controlled by the voice of the person speaking,unless the normally de-energized amplifier can he energizedsubstantially simultaneously with the spoken signal, a portion of suchsignal will be clipped rendering the speech unintelligible. In addition,where such last mentioned system is used, if upon energiza-' tion of oneof said amplifiers and cut-off of the other amplifier, such conditionshould remain as long as the. first speaker is talking, there would beno possibility of the second speaker cutting in or interrupting thefirst speaker and consequently cross conversation, as is possible inconventional telephone communication, is prevented.

It is accordingly among the objects of the invention to provide anintercommunication system that will pick up sounds even at aconsiderable distance from the microphone at one station and amplifysuch sounds so that they may be heard even at a considerable distancefrom the speaker at the other station and whichwill function withoutacoustic feed-back that might render the sounds unintelligible, withoutthe need for any manual. operation on the part of the listener or personspeaking, which does not require the microphone and speaker at therespective stations to be spaced widely apart, which will permit normalconversation between the installation useless for normal ice parties sothat either of. them may interrupt the other at any time. and willdependably reproduce the intelligence or speech without clipping ordistortion thereof.

According to the invention these. objects are obtained by the use of amicrophone at a local station connected to a speaker at a remote stationand a speaker at a local station connected to a microphone at the remotestation, with a speech amplifier associated with each of the microphonespeaker combinations. Each of the speech. amplifiers is normallyde-energized and has a control amplifier associated therewith, and theamplifiers are so connected that upon an audio signal input into one ofthe control amplifiers it will energize the associated speech amplifierand retain the other speech amplifier. de-energized, yet upon an audiosignal input into the second control amplifier of greater amplitude thanthat intothe first control amplifier, the first speech amplifier will becut off and the second speech amplifier energized.

More specifically, according to one aspect ofthe invention, the speechamplifier associated with each of the microphone-speaker combinations isrendered inoperati've'by the lack of sufiicient potential on one of theelements of at least-one of its vacuum tubes. The control amplifierassociated therewith is designed to provide, when actuated by the signalinto the associated microphone, a sufiiciently high potential on suchvacuum tube element to cause the stage in which it is included tofunction, thereby rendering the speech amplifier operative. The controlamplifier is also designed to apply a disabling potential to the outputstage of the control amplifier associatedwith the speech amplifiercontrolling the other microphone-speaker combination so that saidcontrol amplifier will not deliver an energizing potential to saidspeech amplifier and hence the latter will remain inoperative. Thecontrol amplifiers are also so designed, that eventhough the outputstage of one of said control amplifiers may have such disablingpotential applied thereto, as above described, if the input signal tothe output stage of said disabled control amplifier should besufliciently high, such as would occur if the person talking into themicrophone associated with said disabled amplifier should speak in aloud voice, the 'cut-oif' condition of such output stage would beovercome and such output stage would function to provide a voltage whichwould actuate the speech amplifier associated therewith and disable theoutput stage associated with the other control amplifier. As a result,either of the parties" speaking may take, over the conversation by'merelyspeaking in a voice. louder than the other party.

In a specific embodiment herein shown, the speech amplifiers associatedwith each of the microphone-speaker combinations. is renderedinoperative by having the screengrid of the input stage thereofnormally'either' at zero potential or at such a low potential that thetube will not function properly. The screen grid is connected to the 1plate ofzthe' output stage of the control amplifier associated with saidspeech amplifier, said output stage being normally energized so that itsplate voltage by reason of the voltage drop across the associated plateload resistor will be at a relatively low value, not sutficient' fornormal operation of the input tube of the speech amplifier. The output'stage' of the control amplifier is controlled by suitable circuitswhich include an input or amplifier stage fed by the microphoneassociated with the speech amplifier. The output of such input stage isdesirably rectified to produce a negative signal which, when impressedon the grid of the output tube of the control amplifier will cut thelatter ofi; As a result of the sudden decrease inplate current, theplate potential will rise rapidly to a point suflicient to energize theinput stage of the speech amplifier so t-h-atthe latter'will functionproperly. In order'simultaneously to retain the other speech amplifierinoperative,

the plate of the output tube of the first named control amplifier isconnected to the control grid of the output tube of the controlamplifier associated with said last named speech amplifier.Consequently, when the output tube of the first named control amplifieris cut off, a relatively high positive potential will be applied to thecontrol grid of the output tube of the last named control amplifier sothat the latter will continue to conduct thereby preventing anylikelihood of sufficient plate potential being developed by such tubewhich would render operative the speech amplifier controlled thereby.

In the accompanying drawings in which are shown one or more of variouspossible embodiments of the several features of the invention,

Fig. 1 is a detailed circuit diagram of the communication system, and

Fig. 2 is a circuit diagram of a switching arrangement which may beutilized with the system shown in Fig. 1.

Referring now to Fig. l of the drawings for a detailed analysis of thecircuit by means of which the principles of operation of the inventionherein are performed, the system desirably includes a local station,preferably a master station having its own amplifiers. The local stationhas a microphone connected through a conven tional microphonetransformer 11 and lead 12 to the control grid 13 of vacuum tube 14 inthe input stage of the associated speech amplifier 15. Vacuum tube 14which illustratively is a conventional pentode, has its plate 16connected through coupling capacitor 17 to potentiometer 18 and thencethrough the movable arm 19 of such potentiometer to the control grid 22of a vacuum tube 23 which also desirably is a pentode, connected inconventional manner. The plate 24 of tube 23 is connected by lead 25 toa speaker transformer 26, the output of which is connected by leads 27to a speaker 28 at a remote station which illustratively is a secondarystation having no amplifier units associated therewith.

Desirably, one of the elements of the input vacuum tube 14,illustratively the screen grid 32. thereof, is not supplied withsufficient potential and hence the input stage of such amplifier inwhich tube 14 is incorporated, will not function or will function withvery low gain so that the speech amplifier will be inoperative.

Associated with the speech amplifier 15 is the control amplifier 33,which as shown in Fig. 1 has an input tube 34, illustrating a pentodeconnected in conventional manner, the control grid 35 of which isconnected by lead 36 to the output of the microphone transformer 11. Thetube 34, which functions as a conventional pentode amplifier tube, hasits plate 37 connected by lead 38 through coupling capacitor 39 to thepotentiometer 42, the movable arm 43 of which is connected by lead 44 tothe control grid 45 of an amplifier tube 46, illustratively a triodeconnected in conventional manner. The plate 47 of tube 46 is connectedthrough coupling capacitor 48 and lead 49 to the plate 52 of a diodedetector tube 53, the cathode 54 of which is connected to the movablearm 55 of a potentiometer 56, the latter being connected between asource of potential 57 and ground. The plate 52 of diode 53 is alsoconnected by lead 58 to resistors 59 and 62,. which are connectedrespectively by leads 63, 64 and lead 65 to opposite sides of acapacitor 66 and by lead 67 to ground. Lead 65 is also connected by lead68 to the control grid 69 of the output tube 72 of the control amplifier33, said tube 72 illustratively being a triode connected in conventionalmanner as to be normally conducting. The cathode 73 of tube 72 isconnected to the movable arm 74 of a potentiometer 75 which ispositioned between a source of potential 76 and ground. The plate '77 oftube 72 is connected through load resistor 78 to a source of potential79 and also by lead 82 through resistor 83 and lead 84 to the screengrid 32 of tube 14 in speech amplifier 15, a capacitor 86 beingassociated with the resistor 83 in conventional manner to prevent thevarying audio signals from affecting the positive screen grid potential,

As the speech amplifier 87 at the local station is identical to thespeech amplifier 15, it is not shown in detail being diagrammaticallyrepresented by the block outline and has a microphone 99 at thesecondary station asso ciated with the speaker 91 at the local station.Speech amplifier 87, in manner similar to speech amplifier 15, feeds theinput stage of a control amplifier similar to control amplifier 33 anddiagrammatically represented by the block 92 and, through circuitsidentical to those previously described in control amplifier 33 (andbearing the same reference numerals primed), feeds the control grid 93of the output tube 94 of said control amplifier 92.

As is clearly shown in Fig. l, the plate 77 of 'tube 72 is connected bylead 95 through resistor 96 to the control grid 93 of tube 94 and inlike manner the plate 97 of tube 94 is connected by lead 98 throughresistor 99 to the control grid 69 of tube 72. The plate 97 of tube 94is also connected through a plate load resistor 101 to a source ofpotential 102 and by lead 103, resistor 104 and lead 165 to the screengrid (not shown) of the vacuum tube in the input stage of speechamplifier 87.

In the operation of the circuit shown in Fig. 1, when the party at thelocal station talks into microphone 10 a signal will be impressedsubstantially simultaneously on the control grids 13 and 35 of vacuumtubes 14 and 34. As insufficient potential is normally applied to thescreen grid 32 of vacuum tube 14, the speech amplifier 15 will remainsubstantially inoperative. However, as tube 34 is functioning in normalmanner, the signal impressed upon control grid 35 will be amplified andsuch amplifier signal will be applied to the control grid 45 of tube 46,the amount of signal fed to grid 45 being determined by the adjustmentof potentiometer 42.

Tube 46 will also amplify the signal and such signal will be detectedand rectified by the diode 53, which will permit the positive portionsof the signal to pass to ground and the negative portions of the signal,which will be filtered by the network 59, 62 and 66, will be impressedupon the control grid 69 of the normally conducting output tube 72. Tube72 is normally biased at such a point that it will not be cut off byrelatively weak signals, such as from outside noises which may be pickedup by the microphone 16, and impressed on the grid 69 of tube 72 throughthe circuits of the control amplifier above described. The negativeinput to the grid 69 of tube 72, caused by the detection andamplification above described of the voice input, will cause the tube 72to cut ofi. As a result, little or no plate current will fiow throughthe plate load resistor 78 and the voltage on the plate of tube 72 willbe substantially at the applied potential 79. Such high potential willbe impressed through the lead 82, resistor 33 and lead 84 to the screengrid 32 of tube 14 in speech amplifier 15. As a result tube 14 willfunction normally to amplify the signal originally impressed on its grid13 and such amplified signal will be heard through the speaker 28 at theremote station.

As the voltage is applied to the screen grid 32 of tube 14 from thecontrol amplifier 33 almost simultaneously with its input into themicrophone 119, there will be no clipping of the signal and hence nodistortion thereof, for the speech amplifier will be energized by theaction of the associated control amplifier substantially simultaneouslywith the signal input thereto.

1 plate potential to be relatively low. As a result, little or nopotential will be applied to the screen grid of the input amplifier tubein speech amplifier 87 and conse quently the? latter willremaininoperative. Consequently there is little or nolikelihood of acousticfeed-back" being developed by'the' sounds that would, in the absence of:the present. invention", be picked up by the remote microphone 90.

If the party at the remote station should desire to break into theconversation, he need merely talk into-his microphone 90 in a voicelouder than the amplitude at which the first party is speaking. As aresult, the input stage of the control amplifier'92 associated with the"secand speech. amplifier 87 will amplify such loud voice in. the mannerpreviously described with respect-to control amplifier 33 and thesignal: will be detected and rectified by the diode 53 so that anegative signal of relatively great amplitude will be: impressed on thecontrol grid 93 of vacutun tube 94 of the control amplifier 921 Thisnegative: signal will be: sufficient to overcome the positive bias onsuch tube. afiected both through potentiometer 75 and the connectionfrom the platev 77 of vacuum tube 72 to the control grid 93-. As a=result, vacuum tube 94 will be cut off causing. the positive voltage onthe plate 97 thereof to rise rapidly. and? as such positive voltage willbe impressed" on the' control grid 69 of vacuum tube 72throughlead 98"and resistor 99, such positive voltage will tend to make vacuum tube 72conduct heavily as it will overcome the negative rectified signal fromdiode 53; As aresult of the" voltage drop across: plate load resistor 78of vacuum tube 72, the voltage on plate 77 of tube 72 will: drop.vHence, the positive bias through lead-.95 and resistor 96 to controlgrid 93 of tube-94will be reduced, thereby rendering such tube less.conductive until it finally'cuts. off. At this time the voltage on: theplate of tube 94 will be very high and consequently the positive voltageon: the grid 69 of tube 72 will be. sufiicient so: that said tube. willconduct strongly.

It is apparent, therefore, that as the voltage. on the plate of tube 72drops and that on the plate of tube. 94 rises the screen grid voltage ofspeech amplifier will be reduced and such speech amplifier will berendered inoperative. Simultaneously the screen grid voltage of speechamplifier 87 will be increased and such speech amplifier will berendered operative.

Once the second party has takenover and cut oti the first party, thesecond party may then speak in a normal voice for continuedconversation. As the operations above: described occur very rapidly, itis. apparent that as far as the parties are concerned they will. be:maintaining a conversation that approximates almost: a face to facemeeting and either may interrupt the; other. as desired by merelyspeaking in a louder voice.

Itis to be understood that the circuitsherein described could bevincorporated in a systemin which the local station is a master stationas shown having amplifiers associated therewith and having a number ofremote stations. connected to thelocal station, which remote stationsvmay also be master stations havingtheir own amplifiers, or secondarystations without amplifiers.- In such a system it is desirable that theparty at the local: station be able to call one or more of the remotestations with but a few simple manual operations before starting theconversation and thereafter hold a.conversation with the remote stationor stations called. It is also desirable that either the secondary ormaster remote stations be able to call the local master station and thatsuchi'conversation also may be limited by the local master station toonly itself and the station calling.

Although any suitable switching arrangement may be used for thispurpose, it is preferred to use the arrange ment shown in Fig. 2. whichdesirably incorporates the circuits shown in Fig. 1. Thus the masterlocal station has the speech amplifiers 15 and 87andthecontrolamplifiers 33 and 92 associated therewith, the controlamplifiers being diagrammatically represented. by the single blockoutline. Speech amplifier. 15 which has the microphone 10'associated-therewith is connected by leads 111 and 1 12 to contact 113of a double pole double throw switch 114 of the push button type, thecontact 115- associated with contact 113 being connected by lead 116to-terminal117. The speech amplifier87 at the local master station whichhas the speaker 91 associated therewith, has its input connected. bylead to the contact 126 of a single'pole double throw switch 127, theassociated contact 128 being connected by lead 129 to a source of power131 and thence to ground; The movable swit'ch-arm13-3of switch 127,which. is normally retained against contact 126; is

connected by leads 134-and 135 to contact 136 of switch 114 at .themaster station. The contact 137 associated with contact 1361 isconnected by lead' 138 to a terminal 139; Contact137 of'switch 114 isalso connected by lead 141 to contact 142 of switch 114, said contact142 having anassociated contact 143 connected through resistor 144-,lamp 145 and lead 146'to buzzer 147 and then to ground.

Withthe' switching arrangement thus described'a remote master stationmay readily be connected to thelocal master stationiby means oftransmission lines 151' and: 152 connecting the terminals 117, 117 and139, 139'. As the remote master station shown on the right hand side ofFig. 2 is'identical to the'remote-master station shown on theleft handside ofFig. 1' it Will not be described in detail, parts correspondingto the already described local master station having the same referencenumerals primed.

In order that the local master station may be'connected toaremote'secondary'station and that such remote secondary station maycallthe localmaster station, thecircuit shown in the lower'part of Fig.2. is desirably provided.

Thus, a second double pole double throw switch 153 also desirably'of thepushbntton type is'provided at the local station; the contact 154 ofsaid switch beingcom nected. by lead 155 through resistor 156, lamp 157and lead 158 to the junction between lamp 145 and buzzer 147. Thecontact 159 associated with contact 154 is connected. by lead: 161 tocontact 162 of switch 153 and also by lead 163 to terminal 164, thecontact 165 associated with contact 162 being connected by leads 166-and 167 to the junction 168 between leads 134-and 135. The contact 169of. switch 153-is connected by leads 1571 and 17.2 to the junction 173between leads 111 and 112, and the contact 174 associated withcontact'1'69 is connected by lead 175 to terminal 176.

The remote-secondary station shown in Fig. 2 desirably comprises themicrophone 90showu in Fig. 1',.connected inconventional manner andhaving a lead 177 connected to contact178 of a single pole double throwswitch 179; The associated contactt181 of said switch is connected bylead.182 to a buzzer 183 and thence to ground. The movable. arm 184 of.switch 179-which illustratively engages contact 181, is connected bylead 185 to contact 186 of asingle pole double throw switch 187. Themovable arm 188 of switch. 187 which normally engages contact 186 isconnected by lead 189 to terminal 164' of said secondary station. Theother contact 192 of switch 187 is connected by lead 193 to terminal194, and the speaker 28 of said secondary station, which is connected inconventional manner, is connected by lead 195' to terminal 176'.

In the installation of the equipment the remote sec ondary station isconnected to the local master station by means of transmission lines 196and 197 and 198' which connect the terminals 164, 164'; 176, 176and1'94, 194 respectively, the terminal: 194 at the local station beingconnected by lead 199 to lead 129 as at 201.

Although-the local master station has been shown with provision forconnection thereto of. but two remote stations, Le, a master station anda secondary station, it is of course to be understood that any suitablenumber of remote stations, either master or secondary could be con;

nected to the local master station by merely duplicating the switchingararngement shown.

- In the operation of the switching arrangement above described, if theparty at the local master station desires to call the remote masterstation, he need merely press down on the push button of switch 114 sothe movable contact arms 202 and 203 thereof will engage contacts136,137 and 113, 115 respectively. The party at the master station maythen press key 127 so that the switch arm 133 thereof engages contact128. As a result a circuit will be completed from power supply 131through lead 129, contact 128 and switch arm 133, leads 134 and 135,through bridged contacts 156 and 137, lead 138, terminal 139,transmission line 152, terminal 139, leads 138' and 141' through thenormally bridged contacts 142 and 143' of switch 114 at the remotemaster station, through resistor 144 and lamp 145, lead 146', throughbuzzer 147' to ground. As a result, the lamp 145 at the remote stationwill be illuminated and the buzzer 147' will vibrate to notify the partyat the remote station that he is being called. Such party may then pressdown on the push button of switch 114 which will cause the contact arm202 thereof to break the circuit between contacts 142 and 143' and closethe circuit across contacts 136 and 137 and also move the contact arm203 of switch 114 across the contacts 113 and 115' to bridge the latter.As a result, the lamp 145 and buzzer 145 will be cut out of circuit anda circuit will be completed from the terminals 117 and 139' of theremote master station to the speaker and microphone 91' and 10' thereofrespectively. The party at the remote master station may then speak intothe microphone 10' and answer the call. The circuit for microphone 10will be through lead 125', contact 126 of switch 127, contact arm 133'normally engaging said contact 126, leads 134' and 135', n

through the bridged contacts 136' and 137' of switch 114', lead 138,terminal 139', transmission line 152, terminal 139, lead 138, throughthe bridged contacts 136 and 137 of switch 114 at the local masterstation, leads 135 and 134, through movable contact arm 133 and contact126 normally engaged thereby, lead 125, through speech amplifier 87 tospeaker 91 at the local station. The party at the local master stationmay then reply by speaking into the microphone 10 which is connected tothe speaker 91 at the remote master station through the speech amplifier15, leads 111 and 112, bridged contacts 113 and 115, leads 116, terminal117, transmission line 151, terminal 117, lead 116', bridged contacts115 and 113 and leads 112 and 111.

As a result of such switching arrangement a conversation may be heldbetween the parties at the local and remote station, and the amplifiersand 87 with the associated control amplifiers 33 and 92 will function inthe manner previously described to permit the equivalent of face to faceconversation. It is to be noted that with this switching arrangement asthe microphone and speaker of the remote station are normallydisconnected through open switch 114 the party at the master stationwould not be able to hear any sounds from the remote station unless sodesired by the party at the remote station.

If the party at the remote master station desires to call the localmaster station he need merely press the push button associated withswitch 114' and then press the key 127'. As a result, a circuit would becompleted from the source of power 131' at the remote master stationthrough lead 129', contact 128', contact arm 133, leads 134' and 135',bridged contacts 136' and 137 of switch 114, lead 138, terminal 139',transmission line 152, terminal 1'39, leads 138 and 141, normallybridged contacts 142 and 143, resistor 144, lamp 145 and lead 146,through buzzer 147 to ground. The party at the local master stationhearing the buzzer, need merely press the push button associated withthe illuminated lamp 145 to complete the circuit from his microphone 10and speaker 91 to terminals 117 and 139. A conversation may then be heldin the manner previously described.

If the party at the local master station desires to call the remotesecondary station he need merely first press the push button associatedwith switch 153 to move the movable contact arms 211 and 212 thereof tobridge contacts 162, 165 and 169, 174 respectively. He then would pressthe key 127 to move the movable contact arm 133 thereof into engagementwith contact 128. This would complete a circuit from power supply 131through lead 129, contact 128, movable contact arm 133, leads 134, 167and 166, bridged contacts 165 and 162, lead 163, terminal 164,transmission line 196, terminal 164, lead 189, movable contact arm 188in the secondary station, contact 186 which it normally engages, lead185, contact arm 184 illustratively engaging contact 181, through buzzer183 to ground. The party at the remote secondary station then needmerely move contact arm 184 to engage contact 178 and talk intomicrophone 91] which is connected to speaker 91 at the master stationthrough speech amplifier 87.

The key 127 at the local master station ordinarily need not be used tocall the secondary station, for upon pressing of the push button ofswitch 114, for example, at the local master station, a circuit will becompleted to the speaker at the secondary station. However, in the eventthe party at the master station does not wish to call out into hismicrophone he can give warning that he is desiring to call by the use ofsuch key.

It is to be noted that with the circuits above described the party atthe local station cannot eavesdrop on the remote secondary stat-ion forthe latter need merely have the movable contact 184 of switch 179disconnected from contact 178 and engaging contact 181. Thus themicrophone of the secondary station will be out of circuit and hence nosounds can be transmitted from the secondary station to the masterstation until contact arm 184 is moved.

If the party at the secondary station should desire to call the party atthe local master station he need merely press the key 187 so thatmovable contact arm 188 engages contact 192. As a result a circuit willbe completed from the power supply 131 at the local master stationthrough lead 199, terminal 194, transmission line 198, terminal 194,lead 193, contact 192, contact arm 188, lead 189 to terminal 164',transmission line 196, terminal 164, leads 163 and 161, normally bridgecontacts 159 and 154 of switch 153, resistor 156, lamp 157, lead 158through buzzer 147 at the master station to ground.

The party at the master station on hearing said buzzer need merely pressthe push button associated with the illuminated lamp 157 to complete thecircuits heretofore described from microphone 10 and speaker 91 at themaster station to speaker 28 and microphone at the secondary station.

As many changes could be made in the above construction, and manyapparently widely difierent embodiments of this invention could be madewithout departing from the scope of the claims, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

l. A communication system comprising two speech amplifiers, each havingan amplifier tube having a screen grid, two normally conducting controlamplifiers connected respectively to said speech amplifiers, amicrophone designed to receive an audio signal and connected to theinput of each of said speech amplifiers and its associated controlamplifier, a speaker connected to the output of said speech amplifiers,each of said control amplifiers having a normally conducting stagehaving a tube with a plate load resistor, means connecting the plates ofsaid tubes to the screen grids of the associated speech amplifierrespectively, means in each of said control amplifiers, actuated by thesignal into the associated microphone to cut off the normally conductingtube, whereby upon drop of the potential across the associated loadresistor, the voltage on the plate will rise and such potential will besupplied to the screen grid of the amplifier tube of the associatedspeech amplifier to render said tube and said speech amplifieroperative, and means controlled by said rising plate potential of one ofsaid control amplifiers to retain the speech amplifier associated withthe other control amplifier inoperative, whereby upon a signal into oneof said microphones of greater intensity than the signal into the otherof said microphones, the speech amplifier associated with said firstnamed microphone will be rendered operative and the speech amplifierassociated with the second named microphone will be renderedinoperative.

2. The combination set forth in claim 1 in which each of said controlamplifiers comprises a normally conducting input stage, a diode detectorassociated with the output of said input stage and connected to rectifythe signal therefrom and pass the negative portion thereof, the outputof said diode being connected to the control grid of the normallyconducting stage of said control amplifier whereby said negative signalwill cutoff said tube to cause the plate potential thereof to rise.

3. The combination set forth in claim 1 in which the means controlled bysaid rising plate potential to retain said other speech amplifierinoperative and the means to render the speech amplifier associated withsaid first named microphone operative comprises a connection from theplate of the normally conducting stage of one of said control amplifiersto the control grid of the normally conducting stage of the other ofsaid control amplifiers and a connector from the plate of the normallyconducting stage of said last named control amplifier to the controlgrid of the normally conducting stage of said first named controlamplifier.

4. The combination set forth in claim 3 in which each of the connectionsbetween the plate of the normally conducting stage of said controlamplifier and the screen grid of the associated speech amplifierincludes a voltage dropping resistor.

References Cited in the file of this patent UNITED STATES PATENTS2,205,142 Hoard June 18, 1940 2,319,680 Hasenberg May 18, 1943 2,323,307Campbell July 6, 1943 2,347,080 Campbell Apr. 18, 1944 2,515,726 MontaniJuly 18, 1950 2,536,820 Prahm Jan. 2, 1951 2,545,466 Jeanlin Mar. 20,1951 2,545,476 Levy Mar. 20, 1951 2,623,949 Jeanlin Dec. 30, 1952

